A computer network is a collection of interconnected computing devices that can exchange data and share resources. Example network devices include layer two devices that operate within the second layer (L2) of the Open Systems Interconnection (OSI) reference model, i.e., the data link layer, and layer three devices that operate within the third layer (L3) of the OSI reference model, i.e., the network layer. Network devices within computer networks often include a control unit that provides control plane functionality for the network device and forwarding components for routing or switching data units.
An Ethernet Virtual Private Network (EVPN) may be used to extend two or more remote layer two (L2) customer networks through an intermediate layer three (L3) network (usually referred to as a provider network), in a transparent manner, i.e., as if the intermediate L3 network does not exist. In particular, the EVPN transports L2 communications, such as Ethernet packets or “frames,” between customer networks via the intermediate network. In a typical configuration, provider edge (PE) network devices (e.g., routers and/or switches) coupled to the customer edge (CE) network devices of the customer networks define label switched paths (LSPs) within the provider network to carry encapsulated L2 communications as if these customer networks were directly attached to the same local area network (LAN). In some configurations, the PE network devices may also be connected by an IP infrastructure in which case IP/GRE tunneling or other IP tunneling can be used between the network devices.
As the PE network devices in an EVPN forward Ethernet frames, the PE network devices learn L2 state information for the L2 customer networks. The L2 state information may include media access control (MAC) addressing information for the CE network devices and customer equipment within the customer network and the physical ports of the PE network device through which the customer devices are reachable. The PE network devices typically store the MAC addressing information in L2 learning tables associated with each of their physical interfaces. When switching an individual Ethernet frame having a given destination MAC address, a PE network device typically broadcasts the Ethernet frame to all of its physical ports unless the PE network device has previously learned the specific physical port through which to the destination MAC address is reachable. In this case, the PE network device forwards a single copy of the Ethernet frame out the associated physical port.
In an EVPN, MAC learning between PE network devices occurs in the control plane rather than in the data plane (as happens with traditional bridging) using a routing protocol. For example, in EVPNs, a PE network device typically use Multi-Protocol Border Gateway Protocol (MP-BGP) to advertise to other provider edge network devices the MAC addresses learned from the local consumer edge network devices to which the PE network device is connected. A PE device may use MP-BGP route advertisement with EVPN Address Family Identifiers/Subsequent Address Family Identifiers (AFI/SAFI) to announce reachability information for the for one or more MAC addresses learned by the PE network device.
In an EVPN configuration referred to as all-active mode, an Ethernet segment includes multiple PE network devices that provide multi-homed connectivity for one or more local customer network devices. In all-active mode, all PE network devices attached to the Ethernet segment are allowed to forward traffic to and from the Ethernet segment. Moreover, the multiple PE network devices may provide transport services through the intermediate network to a remote PE network device.
When a communication link between one of the multiple PE network devices and a customer network device fails, the PE network device having the failed communication link may withdraw EVPN routes corresponding to the Ethernet segment. The time needed to withdraw from the routing table may be high, especially if the PE network device had a large number of MAC addresses related to the Ethernet segment. During this transient time, any network traffic destined for the customer network device will be undeliverable by the PE network device having the failed communication, and such network traffic may be lost. Similarly, if a communication link between a multi-homed all-active PE network devices has a failed communication link between itself and a core network, any data traffic received from the customer network device destined for the core network will be undeliverable until the broken communication link is re-configured. Again, such re-configuration may take a number of seconds.